A lower olefin (ethylene or propylene) that is an important basic feedstock in petrochemistry is expected to grow in the demand at a steady pace also in the future. Currently, 60% of propylene has been produced by a steam cracking process (steam cracker) of naphtha or the like. However, this technique requires a high temperature of 800 to 900° C. for decomposition because of the absence of a catalyst, and is an energy-intensive process by charging excessive steam.
In addition, the main product of the above-described technique is ethylene, and propylene is produced as a by-product, therefore, in a case where naphtha is used as a feedstock, the production ratio is approximately fixed as ethylene/propylene=2/1. There is a possibility of having a situation that the supply of propylene may not catch up with the expanding demand for propylene in the future. From the viewpoint described above, an alternative process for efficiently producing propylene from a naphtha feeds tock with as little energy consumption as possible is strongly desired.
Currently, as an alternative method of a steam cracker, an energy-saving method for producing propylene, to which a fixed bed-type naphtha catalytic cracking method using a zeolite catalyst is applied, has been researched and developed.
For example, it has been proposed that a crystalline aluminosilicate having a MFI-type structure containing iron or iron and gallium is used as a catalyst when a lower olefin is produced from a low-boiling hydrocarbon feedstock such as light naphtha (see for example, Patent Literatures 1 to 3).
According to the zeolite catalyst described in these Patent Literatures 1 to 3, with a relatively low reaction temperature, the production amount of the propylene relative to the ethylene can be increased, and further the catalyst lifetime can be extended.